How much will a dual layer disc hold? Says 8.5 but not getting anywhere near that

Discussion in 'Mac Basics and Help' started by VoodooDaddy, Oct 5, 2007.

  1. VoodooDaddy macrumors 65816

    May 14, 2003
    I was trying to burn some files to dual layer discs to free up some hard drive space. I am using toast, I dragged what I wanted into toast, it was 8.25 but it said it would span 2 discs. I tried to burn it anyway and sure enough, after a while it prompted me to insert disc 2.

    I then started to delete a few things to see exactly what it would hold and it doesnt appear to me to hold more than 8gb. So why are these discs rated at 8.5 if they wont hold even 8gb?
  2. PlaceofDis macrumors Core

    Jan 6, 2004
    you have to be sure to tell toast that it'll be a dual layer disc prior to burning otherwise it'll assume that its a standard capacity.
  3. VoodooDaddy thread starter macrumors 65816

    May 14, 2003
    Ive got that covered. 7.99 seems to be the limit for an 8.5DL disc. I even tried a burn folder and it said 2 discs needed for 8.25gb.

    Attached Files:

  4. disconap macrumors 68000


    Oct 29, 2005
    Portland, OR
    It's probably that old Bits and Bytes thing; the disc and the OS probably read them separately, and a bit doesn't equal a byte.
  5. Serge N Gin macrumors member

    Serge N Gin

    Sep 29, 2007
    Here is the long version:

    I know you are referring to a Dual-Layer DVD but when you buy a single-layer blank disc, you may see 4.7 GB on it. This is actually a marketing strategy and in actual computing terms, it is not 4.7 GB that should be written. The correct number should be 4700000000 bytes.

    Here is a comparison between marketing and computing definitions: Marketing: 1 KB is equivalent to 1000 bytes Computing: 1 KB is equivalent to 1024 Bytes

    Marketing: 1 MB is equivalent to 1000 KB or 1000000 Bytes Computing: 1 MB is equivalent to 1024 KB or 1048576 Bytes

    Marketing: 1 GB is equivalent to 1000 MB or 1000000000 Bytes Computing: 1 GB is equivalent to 1024 MB or 1073741824 Bytes

    After all these definitions, you can see that marketing people will divide 4700000000 by 1000000000 to obtain 4.7 GB (which is not real)! The correct calculation should be 4700000000 divided by 1073741824 which amounts to 4.37 GB. This is the maximum amount of data that can fit on a single layer DVD.
  6. VoodooDaddy thread starter macrumors 65816

    May 14, 2003
    So 8500000000/1073741824 = 7.91gb. Makes sense. I knew a 4.7gb disc didnt hold a full 4.7, but wasnt sure how much exactly it did hold. Guess I was just surprised to see an 8.5gb disc actually holds less than 8gb.
  7. MikeTheC Guest


    Apr 26, 2004
    Gallifrey -- Capitol City, Prydonian Sector
    Also, don't forget that there's some overhead in there, too, for the filesystem, whatever passes for a FAT table, etc.
  8. HLdan macrumors 603


    Aug 22, 2007
    Actually Toast is intuitive. I just burned a D/L disc last night for the first time on my iMac and Toast automatically detected the blank media was a D/L disc in the drive.

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